/Engine/Source/Runtime/Engine/Public/WorldPartition/WorldPartitionSubsystem.h
Returns true if world partition is done streaming levels, adding them to the world or removing them from the world. When provided, the test is reduced to streaming levels affected by the optional streaming source provider.
bool IsStreamingCompleted ( const [IWorldPartitionStreamingSourceProvider](API\Runtime\Engine\WorldPartition\IWorldPartitionStreamingSourcePr-) * InStreamingSourceProvider ) const